Douglas Carswell says he is still an admirer of David Cameron. He has a funny way of showing it.

If you were surprised to see the now former Conservative MP for Clacton enter a press conference with Nigel Farage, imagine being the Prime Minister.

Mr Carswell didn’t tell the Tories he was leaving — they found out as we did — live on TV.

In fact, even UKIP press officers didn’t seem to know what was about to happen. This really was the best kept secret in politics.

And it’s a potentially fatal blow to David Cameron’s hopes of being re-elected next year.

Douglas Carswell voted for Mr Cameron in the Tory leadership election nine years ago — but has always had a very different attitude to Europe.

A regular Commons rebel on the issue, openly supportive of a British exit from the European Union, it’s no surprise to discover his views hover close to those of UKIP. But defecting is a much bigger move, one that apparently caused him sleepless nights.

While the Tories call his decision “counter-productive”, there remains some admiration inside the party for Mr Carswell, especially as he’s chosen to resign his seat and stand again as a UKIP candidate rather than just switch sides from within the Commons.

Who, then, will keep the Tory promise to field a strong candidate against him? Who would want to oppose a man who, until a few hours ago, was a card-carrying Conservative?

Mr Carswell goes into this by-election with a strong majority — more than 12-thousand, and in an area described by one pollster as the most UKIP-friendly part of the country.

In the next few weeks, we’ll see how much of that is a party vote, and how much is personal support.

Could there be more defections?

It’s entirely possible. UKIP’s often hinted it was talking to a few MPs about making the switch, and Nigel Farage continues to hint there are other Tories — maybe even some on the Labour benches, who could be persuaded to jump.

But while it’s great publicity for UKIP, Douglas Carswell’s defection could actually make a British exit from the EU far less likely.

As David Cameron has repeatedly said, the Conservatives are the only party promising an in/out referendum, and unlike UKIP stand a reasonable chance of delivering that by remaining in office.

And the biggest obstacle to that? UKIP.

Every Conservative voter lured to UKIP is another step towards Downing Street for Ed Miliband, who has no intention of taking Britain through Europe’s exit door. Nigel Farage’s rise could in fact more firmly cement Britain’s position in the EU.

As we head into the conference season, this is exactly what David Cameron did not want.

Instead of a platform for the Tories to start their push for an outright majority next May, now they must fight one of their own, waving the flag of a rival. If Douglas Carswell is sent back to the Commons as a UKIP MP, which seems very likely, it may encourage others to take the same leap of faith. This could be the beginning of a very uncomfortable period for Mr Cameron.

If nothing else, it will once again put the Conservative Party’s never-ending battle over Europe in the spotlight, drown out any other issue they try to raise, and make the job of keeping the Tories in office that bit harder.